British police investigating Friday's sword attack outside Buckingham Palace in London arrested a second man on Sunday, AFP reported.
"The man was arrested on suspicion of being involved in the commission, preparation or instigation of terrorism at 10:15 hrs (0915 GMT) today and he has been taken into custody," a police statement said.
It came hours after Islamic State (ISIS) jihadists claimed responsibility for Friday's knife attack on patrolling soldiers in Brussels, and as authorities in Spain revealed that the death toll from last week's twin attacks in Barcelona had risen to 16.
British police said they had arrested a 30-year-old man at an address in West London and were searching a property as part of the inquiry.
A warrant for the extended detention of the 26-year-old man involved in Friday's incident has also been granted, the statement said, according to AFP.
The suspect, who was arrested under the Terrorism Act, had reached for a four-foot sword upon being challenged by unarmed police officers after deliberately driving at them outside the royal palace.
Police said the man, who comes from Luton, located some 30 miles (50 km) north of London, repeatedly shouted "Allahu akbar" and was incapacitated with CS gas.
London was targeted by a terrorist attack in June, in which three terrorists drove into passengers with a van on the London Bridge, then proceeded to stab people at the nearby Borough Market, where they were eliminated by security forces.
In April, a terrorist was arrested near Parliament before he was able to carry out an attack.
35 people have been killed in three jihadist attacks in London and Manchester since March.
Much of Europe remains on high alert following a string of deadly assaults over the past two years.
Last week, Spain was hit by twin vehicle attacks -- on Las Ramblas in Barcelona and in the seaside resort of Cambrils -- which left 16 dead and around 120 wounded.
The Belgian man of Somali origin who attacked patrolling soldiers in Brussels with a knife on Friday died of his injuries after being shot and was later described by Islamic State as one of its "soldiers".